Paula Cleggett-Haleim
Headquarters, Washington, D.C.
January 4, 1993
(Phone: 202/358-0883)
11:30 A.M. EST
Jim Elliott
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-6256)
Release: 93-1
MYSTERIOUS CONCENTRATION OF DARK MATTER DISCOVERED
Astronomers have discovered a huge concentration of mysterious
"dark matter" using the international ROSAT X-ray observatory.
The discovery appears to confirm previous suggestions of where
most of the dark matter in the universe may be concentrated, namely
in and around small groups of galaxies, according to astronomers.
"The new findings add much weight to the theory that most of the
mass of the universe consists of dark matter, the precise nature of
which remains unknown to scientists," said John S. Mulchaey of the
Space Telescope Science Institute, Baltimore, Md., and the
University of Maryland, College Park.
Dark matter is believed to exist although it has never been seen
because it emits no radiation. Its existence has been inferred
because fluctuations observed in the Big Bang -- the explosion
presumed to have created the universe -- did not have sufficient
gravitational pull to cause ordinary matter to coalesce immediately.
It is presumed, therefore, that unknown, or dark, matter that would
be attracted to the weak gravity of the fluctuation got the process
started.
Scientists also presume the existence of the invisible material
because the speeds with which certain galaxies are moving, their
rotational patterns and their shapes cannot be accounted for by the
possible gravitational pull of observable matter close enough to
influence them.
Dark matter may, in fact, constitute up to 95 percent of the mass
of the universe. Confirming its existence and volume would mean
that there might be enough mass in space to "close the universe."
This means that eventually the expansion of the universe, which is
being slowed by the pull of gravity, would come to a halt or nearly
so.
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The discovery announced today was made with x-ray pictures of
three galaxies known as the "NGC 2300 group," located about 150
million light-years from Earth in the direction of the northern
constellation Cepheus. (One light-year is the distance light
travels in 1 year, approximately 5.8 trillion miles.)
The images were taken with the Position Sensitive Proportional
Counter instruments on ROSAT during April 25-27, 1992, according to
Dr. Richard F. Mushotzky, NASA Goddard Space Flight Center,
Greenbelt, Md. They show that the small group of galaxies is
immersed in a huge cloud of hot gas, about 1.3 million light-years
in diameter, he explained.
500 Billion Times The Sun
Astronomers estimate that the cloud has a mass equal to 500
billion times that of the sun and is at a temperature of
approximately 18 million degrees Fahrenheit (10 million degrees
Kelvin).
"A cloud like this would have dissipated into space long ago,
leaving nothing for us to detect, unless it was held together by the
gravity of an immense mass," Mushotzky said. "The mass required to
restrain the cloud is about 25 times greater than the mass of the
three galaxies that are present."
This is the first time that a multimillion degree gas has been
found to pervade a small group of galaxies, the Goddard astronomer
explained, although such gas has been detected in larger clusters of
galaxies by earlier satellite telescopes such as NASA's High Energy
Astronomy Observatory-2.
Results from the Hubble Space Telescope and other satellites
already have shown that if the leading version of the Big Bang
theory is correct, then 90 to 95 percent of the mass in the universe
must be in the unknown "dark" form, astronomers explain. This
means, they say, that there must be 10 to 20 times as much dark
matter by mass as ordinary matter, which scientists call "baryonic
matter."
However, in locations observed previous to this work, the ratio
of dark to ordinary matter has been, at most, a factor of two to
four. Much of that work concentrated on studies of the most
prominent groups of galaxies in space, the "rich clusters,"
according to astronomers. "Rich clusters" are huge aggregates of
hundreds to thousands of galaxies.
"Although they stand out the best and therefore, have been the
subject of most of the research on dark matter, they are not
representative of the universe, because most galaxies are in small
groups like the NGC 2300 group," Mulchaey explained.
"The universe is like the pre-industrial United States, in which
the most conspicuous population concentrations were in a few big
cities, but in which most people actually lived in small towns and
rural America," he said.
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If small groups of galaxies all have comparable ratios of dark to
ordinary matter, meaning a factor of 12 to 25 as found in the NGC
2300 group rather than a factor of four or less as found in rich
clusters of galaxies, Mulchaey explained, then the mystery of where
most of the dark matter in the universe is located has been solved.
Closed Universe
That would mean that there might be enough mass in space to
"close the universe," indicating that eventually the expansion of
the universe, which is being slowed by the pull of gravity, would
come to a halt or nearly so, he continued.
Although some scientists have suggested that the dark matter
might be preferentially concentrated in small groups of galaxies,
direct evidence was lacking until ROSAT observations were made,
according to the astronomers. Further work is needed to confirm a
discovery of this apparent magnitude, they admit.
This work represents the first case in which the amount of dark
matter in a small group of galaxies has been determined accurately,
the science team reported.
"For confirmation, we need repeated x-ray observations from space
of the NGC 2300 group and other representative small groups of
galaxies," Mushotzky explained. "Meanwhile, if we are right, the
theorists need to start thinking about why there is much dark matter
where there is little ordinary matter (i.e. in small groups of
galaxies), and there is much less dark matter where there is a lot
of ordinary matter (in rich clusters of galaxies)," he said.
The discovery is to be announced today at a meeting of the
American Astronomical Society in Phoenix, Ariz., by Mulchaey; David
S. Davis, NASA Goddard Space Flight Center and the University of
Maryland; Dr. Richard F. Mushotzky; and Dr. David Burstein, Arizona
State University, Tempe.
Mulchaey and Davis are University of Maryland graduate students
who work with Dr. Mushotzky's research group at the Goddard Space
Flight Center. Mulchaey currently is employed at the Space
Telescope Science Institute. Dr. Burstein is a Professor of
Astronomy at Arizona State.
ROSAT, an acronym for Roentgen Satellite, is a joint project of
Germany, the United States and the United Kingdom. It was launched
on a Delta II rocket from the Cape Canaveral Air Force Station,
Fla., on June 1, 1990.
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EDITORS NOTE: A video and photograph to illustrate this story is
available by calling NASA's Broadcast and Imaging Branch on
202/358-1741.
Color B&W
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